It is known that molybdenum trioxide is considered to be the most important molybdenum compound. In commerce, three common grades of MoO.sub.3 are the Technical Grade (approximately 90% of more MoO.sub.3), Grade B (approximately 98% MoO.sub.3) and Pure Grade (approximately 99.9% MoO.sub.3).
The reduction of molybdic oxide (molybdenum trioxide MoO.sub.3) to metallic molybdenum has been the subject of considerable investigation. For example, in the November 1964 Journal of Metals, A. B. Michael and J. B. Hanway, Jr. pointed out the following: p1 "The hydrogen reduction of molybdic oxide has been demonstrated to occur in stages. During the reduction, molybdic oxide successively passes through several lower oxides and eventually metallic molybdenum is produced. The temperatures required for practical degrees of production progressively increase as the lower oxides are formed. For simplicity, however, the reduction may be considered to take place in two stages: (1) molybdic oxide (MoO.sub.3) is reduced to molybdenum dioxide (MoO.sub.2) at a temperature of approximately 500.degree. C., and (2) molybdenum dioxide (MoO.sub.2) is reduced to molybdenum metal at temperatures as low as 750.degree. C.; a more practical temperature for the final state of reduction, however, is about 1000.degree. to 1100.degree. C."
The authors then proceeded to describe their development and testing of a single-stage fluid bed process for converting MoO.sub.3 to Mo metal. Their process sought to retain the heat generated in the exothermic first stage of reduction within the reactor so that heat required to preheat the fluidizing hydrogen to accomplish the endothermic second stage of reduction would be kept within practical limits. It was postulated that the MoO.sub.3 fed to the reactor would become molten enough to attach itself to the original bed particles before or while being reduced to the dioxide. It was considered this would result in general buildup or growth of bed particles so that the final molybdenum product would be granular. Michael et al. found an operating temperature in their single-stage bed approaching 955.degree. C. was preferred. It is known, however, that at temperatures above 650.degree. C., MoO.sub.3 will sublime causing the bed to get sticky and eventually defluidize. U.S. Pat. Nos. 2,398,114; 2,987,932; 3,264,098; 3,865,573 and U.S. Pat. No. 4,045,216 can also be mentioned. In U.S. Pat. No. 2,398,114, a boat-and-tube furnace was used and batches of water-granulated MoO.sub.3 were treated therein stage-wise with the first stage being conducted at a temperature not substantially exceeding 630.degree. C. in an atmosphere of dilute reducing gas which could be hydrogen, carbon monoxide, ammonia or mixtures with sufficient dilution of the reducing gas with diluents such as steam, nitrogen, or carbon dioxide to control the temperature rise in the exothermic first stage. The second stage reduction to molybdenum metal was then conducted in hydrogen at the higher temperature of about 1040.degree. C. U.S. Pat. No. 2,987,392 is directed to the reduction of MoO.sub.3 to molybdenum metal in a fluid bed which can be either single-stage or multi-stage using hydrogen as the reducing gas. U.S. Pat. No. 4,045,216 is directed to a continuous process for producing a dense pelletized metallic molybdenum product from pelletized molybdenum trioxide feed material in a vertical reactor using hydrogen as the principal reducing agent wherein, in a first stage molybdenum trioxide is reduced to molybdenum dioxide at preferably 600.degree. to 640.degree. C. in hydrogen which is diluted with nitrogen and water vapor and the second stage reduction of molybdenum dioxide to molybdenum is conducted at a temperature exceeding 900.degree. C. using a gas richer in hydrogen than that used in the first stage. U.S. Pat. No. 3,865,573 is directed to the stepwise reduction of molybdenum trioxide to molybdenum dioxide at 500.degree.-600.degree. C. followed by reduction of the dioxide to metal at 800.degree.-90020 C. Hydrogen, reformed gas or cracked ammonia are used as the reducing gas. Feed for the process is briquetted with iron or iron oxide powder to provide a metallized ferromolybdenum briquette for addition to molten steel. The patent notes that impurities merely pass through the process. U.S. Pat. No. 4,547,220 is directed to the reduction of molybdenum trioxide to molybdenum dioxide in a rotary kiln using ammonia as a reductant at a temperature of 400.degree. to 500.degree. C. U.S. Pat. No. 4,659,376, assigned to the same assignee as is the present application is directed to two-stage fluid bed reduction of molybdenum trioxide to molybdenum metal using ammonia as the fluidizing-reducing gas at 400.degree. to 650.degree. C. in the first state and hydrogen as the fluidizing-reducing gas at 700.degree. C. to 1400.degree. C. in the second stage. Significant reduction of impurities, particularly lead and zinc, is obtained.
The art recognizes that the reduction of MoO.sub.3 to Mo metal is preferably conducted in stages to yield MoO.sub.2 as the intermediate product, with separately controlled atmospheres and temperatures for each stage and using various processing procedures including reactors handling briquetted feed, the rotary kiln and the fluid bed. Both single stage and multi-stage operation are contemplated as well as the use of both static and moving beds of material.
Prior work by the present inventors indicated that defluidization of the bed seemed to occur more frequently during reduction of dioxide prepared from pure trioxide than it did when Grade B was the starting material. Also, Grade B oxide caused more defluidization problems than technical oxide. Since these materials contain increasing amounts of gangue (primarily aluminates and silicates), it was felt that these materials somehow aided fluidization.
Further study of the fluid-bed reduction of MoO.sub.3, particularly of technical grade MoO.sub.3 (90% or more MoO.sub.3) has now demonstrated that improvements in the reduction process which result in materially and beneficially increasing the flowability of the Mo metal can be achieved by modifying material feeds and reduction conditions.